They can work 2 ways really, they either plug into the air-intake and lie to the ECU about the amount of air available or they plug into the injector pressure sensor and lie to the ECU about the fuel pressure.

I've also seen them lie to the ECU about engine temprature causing the ECU to think the engine is still cold and overfueling the car.

I've heard mixed reviews of them, I've tried them myself aswell recently, I did notice some gains but I've opted to return it get a remap instead from a reputable company as the gains were far too ambitious and MPG didn't improve really at all.

Lenny

October 13, 2011, 11:00 am

As said above mate,
go for a Blue Fin remap or similar better for your engine and turbo